DURHAM — An expert on terrorism at the University of New Hampshire says it's still too early to gauge the motives of the person or group responsible for the bombings in Boston this week.

Cesar Rebellon, an associate sociology professor, has cautioned students in his class on terrorism to be cautious about jumping to conclusions about the motives behind the act.

Rebellon, a criminology expert who has been lecturing on terrorism at UNH since 2005, said he was in class on Monday when a student alerted him to events at the Boston Marathon, where two bombs detonated shortly before 3 p.m. Three people were killed and more than 170 others were wounded.

While it's plausible to believe the bombing was carried out by a foreign group with political aims, Rebellon said he has no reason to rule out the possibility a domestic group or a lone individual seeking attention is responsible.

President Barack Obama pronounced the deadly Boston Marathon explosions an act of terrorism on Tuesday as individuals close to the investigation said the two bombs were made of pressure cookers packed with ball bearings and metal shards that cut into the victims. Sources told The Associated Press the bombs were placed inside black duffel bags on the ground near the finish line of the annual race.

Obama said investigators “don't have a sense of motivation yet” as they begin to evaluate the attack.

“It would not surprise me if what they wanted to do was get as much attention as possible,” Rebellon said. “Whether that's attention that they want because they feel like they haven't been heard in the past, whether that's attention that they want so as to change some policy or to scare people into changing some policy ... (it's) hard to say. It's impossible to say at this point.”

With many affected by the bombing still searching for answers, Rebellon said it's important to remember past examples when gut reactions to a horrific event proved unreliable. For example, in Norway in 2011, some initially suspected the terrorist group al-Qaida was responsible for carrying out a bombing that killed several people. In reality, it was a lone right-wing extremist who was responsible. He went on to shoot dozens more innocent victims on an island after the bombing.

Rebellon says it's also hard to draw conclusions from the fact no group has claimed credit for the attack in Boston. While the culprits could be waiting to identify themselves, Rebellon said it's also possible that an amateur carried out the bombing, then became concerned about apprehension.

Another factor that complicates the investigation is the fact that terrorist groups have tended to become more decentralized in the last few decades. As a result, some offshoot groups are working in isolation, sharing only a common goal with a larger network of operatives.

Even inside an organization with a hierarchy, the goals of the people who execute attacks can differ. In the case of al-Qaida, leader Osama Bin Laden articulated a relatively consistent set of grievances about the values and foreign policy of the United States. But the men who executed the group's attacks often expressed different motives. In some cases, they were simply frustrated and unhappy with life.

While the investigation progresses, Rebellon said investigators will likely look for significance in the type of explosives that were used, as well as factors such as the scope of the blast, the amount of smoke that was produced and the context of where the bombs were placed along the marathon route.

Security was more evident at sites across the U.S. following Monday's carnage. Military personnel were seen near the Pentagon's subway station in Washington, D.C., and officers deployed to Chicago's Union Station.

At the White House, the Secret Service expanded its security perimeter after the attacks, shutting down Pennsylvania Avenue and cordoning off the area with yellow police tape.

In New York, authorities deployed highly visible patrol units that move in packs with lights and sirens along with more than 1,000 counterterrorism officers. Highly trafficked tourist landmarks were being especially monitored.

“No matter how many days, months or years pass without a major terrorist attack, it only takes one such attack to bring us back to the cruel reality,” Interpol chief Ron Noble told The Associated Press early Tuesday, saying police around the globe would be on alert.

U.S. Secretary of Homeland Security Janet Napolitano urged the American public “to be vigilant and to listen to directions from state and local officials.”

Twelve years ago, UNH professor Charles Putnam found himself in the role of one of those state officials. Putnam was heading the criminal justice bureau at the New Hampshire Attorney General's office at the time of Sept. 11 attacks in New York. In the days that followed, Putnam said he and others wrestled with questions about how the state should respond.

Usually, law enforcement works at the state level, but when the federal government becomes involved, states often relinquish their jurisdiction over an investigation. This becomes more complicated in the case of mass casualty incidents, which often leave state officials on alert for the potential of a continuing threat.

Putnam says tighter cooperation appears to have developed between federal officials and their counterparts on the state level since the terrorist attacks of 2001. Federal investigators have also become more skilled at deciding which information is appropriate to release as an investigation progresses, he said.

“People had to make decisions without perfect or even particularly good information...” he said, remembering the events of 12 years earlier. “I think that we've had some situations where federal officials shared, perhaps, the wrong kind of information, and then other times where they didn't share information with state officials who had important jobs to perform as well.”

As the investigation in Boston continues, Putnam said one issue that's likely being debated now is the extent to which the investigation is conducted under state criminal law, versus the auspices of federal laws that apply to investigations into international terrorism. The question will be especially relevant if a case reaches the court system.

Reflecting on the unseen work that takes place during a terrorism investigation, Putnam said the public should remember that identifying those who are responsible is often a painstaking effort.

“I wish that more members of the media and the public understood that government officials cannot ... predict the future with certainty, and even when they're working very, very hard to understand an event like this, they probably have a lot of questions themselves,” he said. “The procedurals on television persuade many of us that these problems are the kind of things that should get solved in an hour television show. In real life, that's just not what's happening.”